The treatment of cataract, as the world's most common cause for blindness, is a well known process since the time of ancient Rome (first and second century AD). Since that time, the complete removal of the opaque human lens is still the best choice to partially restore visual acuity of the patient. The achieved results are unexpectedly poor because of the disregarded refractive contributions of the natural human lens to the visual apparatus which are not adequately compensated in this situation.
A breakthrough in cataract surgery was made in 1949 when the English physician Harold Ridley successfully implanted the first intraocular lens made of hard PMMA plastics. This lens was capable to compensate for lost optical power of the natural human lens. Since this time IOLs and surgical techniques were continuously improved. Today cataract surgery is by far the most performed surgery in ophthalmology with more than 2.3 million patients per year in the United States and approximately another 3 million surgeries in Europe and Japan.
The capacity of the human eye as an optical system can only be accomplished if the artificial lens is properly positioned and focused. If this condition is satisfied, the incident rays from distant object points form only minimally blurred spots at the retina and provide sharp vision. The correct adaptation of an IOL to the individual human eye remains difficult and the postoperative visual acuity of the patient depends on several factors.
Inaccuracies during measurement of the various ocular geometries, inaccuracies during surgery and postsurgical effects (such as surgical trauma and wound healing processes) limit the achievable visual acuity due to positioning errors of the implanted IOL. Positioning errors with respect to the optical axis mainly cause defocusing while tilt and decentration of the IOL will result in induced astigmatism and coma errors. Higher-order optical aberrations will appear as well.
Up to the present time, different IOL design approaches deal with these problems and try to mitigate the problems with particular emphasis on certain aspects.
A selection of prior art lens designs is described in brief hereinafter.
The equi-convex lens design (example Bausch & Lomb LI61U) is the most used intraocular lens design in clinical practice. Both lens surfaces are spherical with equivalent radii of curvature. As a consequence, these designs produce a significant amount of spherical aberration. Due to the strong increase of spherical aberration with increasing pupil diameter, the patients will very likely suffer from blurry vision and contrast loss under mesopic/scotopic conditions.
The biconvex or plano-convex lens (example AMO sensar AR40) is another lens design. The additional degrees of freedom allow designing a “best shaped IOL” that provides minimal spherical aberration that is achievable with spherical surfaces. The amount of spherical aberration is significantly reduced as compared with the above lens. Since the amount of spherical aberration (SA) is still higher than with the natural human lens, the patient will very likely suffer from blurry vision and contrast loss under mesopic/scotopic conditions due to spherical aberration.
A wavefront optimized IOL (example Pharmacia, TECNIS 29000) is described in U.S. Pat. No. 6,609,793 B2. The anterior surface is aspherical. The deviations from the base sphere are expressed as a sixth order polynomial expansion. The IOL design is based on averaged wavefront aberrometry data obtained on a large patient cohort. The objective of the aspherization is to compensate for the positive spherical aberration as induced by the normal human cornea. The IOL has to provide a certain amount of negative spherical aberration to bring the entire optical apparatus to zero spherical aberration. As viewed from a theoretical optics perspective, this design should provide maximum optical performance at the narrowest possible point spread function. The lens TECNIS 29000 provides a diffraction limited optical performance in the axis-near region. This holds true even for large pupil diameters of 6 mm. Such lens design, however, has also some disadvantages. Due to the intended significant negative spherical aberration of the lens, the latter becomes very sensitive with respect to decentration that is likely to occur during the implantation and after implantation during capsular bag symphysis. The diffraction limited performance of the lens vanishes immediately even if slightly decentered.
The “aberration-free IOL” (example Bausch and Lomb, SofPort A0 and Akreos AO) is disclosed in United States patent publication US 2005/203619 A1 and WO 2004/090611 A3. Both surfaces of the IOL are aspherical and the shape is defined by a conic constant. Considering the specific optical conditions behind the cornea, the IOL does not introduce any additional spherical aberration into the optical system. In other words, the IOL is “transparent” for the incoming aberrations. Systems that do not introduce spherical aberrations do not introduce coma while decentered. Therefore, these lenses can be significantly decentered without losing contrast when compared to the perfectly centered state. Since the spherical aberration of the cornea is not affected by the IOL, this amount of spherical aberration is manifest and limits the optical performance in the axial region. The “aberration-free IOL” does not correspond to the physiological properties of the natural human lens and therefore can lead to sub-optimal results. This lens can be used for eyes after refractive surgery, eyes with keratoconus or with atypical corneal spherical aberration.
There are several other patent publications directed to the subject of increasing the spherical aberrations in order to provide depth of field or achieve pseudoaccommodation.
In United States patent publication US 2004/0230299 (Nov. 18, 2005), an oscillating surface superimposed on a spherical surface is provided to produce different focus points forward and rearward of the best focus in order to obtain an increased depth of focus.
Patent publication WO 2005/046527 (May 26, 2005) discloses a multizone monofocal lens. Each zone presents a positive or negative gradient of refractive power proceeding from the base power of the lens in order to produce an extended depth of field.
U.S. Pat. No. 6,126,286 (Oct. 3, 2000) discloses a multizone monofocal lens to produce an improved depth of field.
European patent publication 1 402 852 (Sep. 29, 2003) discloses a monofocal aspherical lens which permits a pseudoaccommodation by providing an improved depth of field (by increasing the amount of spherical aberrations).